I have two tachometers (one NOS) and both are not working. One just jumps around a lot and doesn't have a steady reading and the other maxes out once the key is turned to on. I've opened both of them up and noticed nothing broken or disconnected. Even tried resoldering all the connection for the one that jumps around and nothing has helped. I know that Randy York was repairing them using 200SX boards but he is not around anymore (RIP). I might try the attempt myself but would rather pay someone else whose done it several times. Is there anyone who does this on the side?

Because when you spend a silly amount of money on a silly, trivial thing that will help you not one jot, you are demonstrating that you have a soul and a heart and that you are the sort of person who has no time for Which? magazine. – Jeremy Clarkson

I would bet some $$ it's not that tach. I've personally never ran across a "broken" factory tach. Yes some slow and off by a few hundred RPM's but not broken aka not able to get them to hold a steady idle. Sounds to me like a wiring issue. If your main power input to the chassis harness is shitty it will make the tach jump around. Check the white wire and make sure the connections are solid. Also clean the fuse box with some vinegar.

For reference we once got a free two door that a tree fell on, it sat in a field for years with no glass. The tach was pulled and installed in another car. It was lagging by 3 but worked! Super funny to rev and watch

"People don't like it when shit doesn't match their rule of thumb." Sam

Grounded the distributor and even tried the 1k ohm resistor that someone suggested. Wiring looks very good and the fact that both tachs will allow the car to start while responding differently makes me think it's not just the wiring. But going to see if I can ground the NOS tach tonight and see if that makes a difference.

I fought with the same problem with an aftermarket tach for over a year. I replaced a lot of wires before the problem was found. It turned out to be the mail power wire for the car that hooks to the starter and goes to the fuse box was my problem. A bad crimp connector was all it was. Make sure you check the wires going to your coil also.

If no one from the future comes back to stop you from doing it then how bad of a decision can it really be?